Metallization of solar cells

ABSTRACT

Approaches for the metallization of solar cells and the resulting solar cells are described. In an example, a method of fabricating a solar cell involves forming a barrier layer on a semiconductor region disposed in or above a substrate. The semiconductor region includes monocrystalline or polycrystalline silicon. The method also involves forming a conductive paste layer on the barrier layer. The method also involves forming a conductive layer from the conductive paste layer. The method also involves forming a contact structure for the semiconductor region of the solar cell, the contact structure including at least the conductive layer.

TECHNICAL FIELD

Embodiments of the present disclosure are in the field of renewableenergy and, in particular, include approaches for the metallization ofsolar cells and the resulting solar cells.

BACKGROUND

Photovoltaic cells, commonly known as solar cells, are well knowndevices for direct conversion of solar radiation into electrical energy.Generally, solar cells are fabricated on a semiconductor wafer orsubstrate using semiconductor processing techniques to form a p-njunction near a surface of the substrate. Solar radiation impinging onthe surface of, and entering into, the substrate creates electron andhole pairs in the bulk of the substrate. The electron and hole pairsmigrate to p-doped and n-doped regions in the substrate, therebygenerating a voltage differential between the doped regions. The dopedregions are connected to conductive regions on the solar cell to directan electrical current from the cell to an external circuit coupledthereto.

Efficiency is an important characteristic of a solar cell as it isdirectly related to the capability of the solar cell to generate power.Likewise, efficiency in producing solar cells is directly related to thecost effectiveness of such solar cells. Accordingly, techniques forincreasing the efficiency of solar cells, or techniques for increasingthe efficiency in the manufacture of solar cells, are generallydesirable. Some embodiments of the present disclosure allow forincreased solar cell manufacture efficiency by providing novel processesfor fabricating solar cell structures. Some embodiments of the presentdisclosure allow for increased solar cell efficiency by providing novelsolar cell structures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A illustrates a cross-sectional view of a portion of a solar cellhaving contact structures formed on emitter regions formed above asubstrate, in accordance with an embodiment of the present disclosure.

FIG. 1B illustrates a cross-sectional view of a portion of a solar cellhaving contact structures formed on emitter regions formed in asubstrate, in accordance with an embodiment of the present disclosure.

FIGS. 2A-2C illustrate cross-sectional views of various processingoperations in a method of fabricating solar cells having contactstructures, in accordance with an embodiment of the present disclosure.

FIG. 3 is a flowchart illustrating operations in a method of fabricatinga solar cell, in accordance with an embodiment of the presentdisclosure.

FIGS. 4A-4D illustrate cross-sectional views of various processingoperations in another method of fabricating solar cells having contactstructures, in accordance with an embodiment of the present disclosure.

FIGS. 5A-5E illustrate cross-sectional views of various processingoperations in another method of fabricating solar cells having contactstructures, in accordance with an embodiment of the present disclosure.

FIG. 6A illustrates cross-sectional views of various processingoperations in another method of fabricating solar cells having contactstructures, in accordance with an embodiment of the present disclosure.

FIG. 6B illustrates cross-sectional views of various processingoperations in another method of fabricating solar cells having contactstructures, in accordance with an embodiment of the present disclosure.

FIGS. 7A-7D illustrate cross-sectional views of various processingoperations in another method of fabricating solar cells having contactstructures, in accordance with an embodiment of the present disclosure.

DETAILED DESCRIPTION

The following detailed description is merely illustrative in nature andis not intended to limit the embodiments of the subject matter or theapplication and uses of such embodiments. As used herein, the word“exemplary” means “serving as an example, instance, or illustration.”Any implementation described herein as exemplary is not necessarily tobe construed as preferred or advantageous over other implementations.Furthermore, there is no intention to be bound by any expressed orimplied theory presented in the preceding technical field, background,brief summary or the following detailed description.

This specification includes references to “one embodiment” or “anembodiment.” The appearances of the phrases “in one embodiment” or “inan embodiment” do not necessarily refer to the same embodiment.Particular features, structures, or characteristics may be combined inany suitable manner consistent with this disclosure.

Terminology. The following paragraphs provide definitions and/or contextfor terms found in this disclosure (including the appended claims):

“Comprising.” This term is open-ended. As used in the appended claims,this term does not foreclose additional structure or steps.

“Configured To.” Various units or components may be described or claimedas “configured to” perform a task or tasks. In such contexts,“configured to” is used to connote structure by indicating that theunits/components include structure that performs those task or tasksduring operation. As such, the unit/component can be said to beconfigured to perform the task even when the specified unit/component isnot currently operational (e.g., is not on/active). Reciting that aunit/circuit/component is “configured to” perform one or more tasks isexpressly intended not to invoke 35 U.S.C. §112, sixth paragraph, forthat unit/component.

“First,” “Second,” etc. As used herein, these terms are used as labelsfor nouns that they precede, and do not imply any type of ordering(e.g., spatial, temporal, logical, etc.). For example, reference to a“first” solar cell does not necessarily imply that this solar cell isthe first solar cell in a sequence; instead the term “first” is used todifferentiate this solar cell from another solar cell (e.g., a “second”solar cell).

“Coupled”—The following description refers to elements or nodes orfeatures being “coupled” together. As used herein, unless expresslystated otherwise, “coupled” means that one element/node/feature isdirectly or indirectly joined to (or directly or indirectly communicateswith) another element/node/feature, and not necessarily mechanically.

In addition, certain terminology may also be used in the followingdescription for the purpose of reference only, and thus are not intendedto be limiting. For example, terms such as “upper”, “lower”, “above”,and “below” refer to directions in the drawings to which reference ismade. Terms such as “front”, “back”, “rear”, “side”, “outboard”, and“inboard” describe the orientation and/or location of portions of thecomponent within a consistent but arbitrary frame of reference which ismade clear by reference to the text and the associated drawingsdescribing the component under discussion. Such terminology may includethe words specifically mentioned above, derivatives thereof, and wordsof similar import.

Approaches for the metallization of solar cells and the resulting solarcells are described herein. In the following description, numerousspecific details are set forth, such as specific process flowoperations, in order to provide a thorough understanding of embodimentsof the present disclosure. It will be apparent to one skilled in the artthat embodiments of the present disclosure may be practiced withoutthese specific details. In other instances, well-known fabricationtechniques, such as lithography and patterning techniques, are notdescribed in detail in order to not unnecessarily obscure embodiments ofthe present disclosure. Furthermore, it is to be understood that thevarious embodiments shown in the figures are illustrativerepresentations and are not necessarily drawn to scale.

Disclosed herein are methods of fabricating solar cells. In anembodiment, a method of fabricating a solar cell involves forming abarrier layer on a semiconductor region disposed in or above asubstrate. The semiconductor region includes monocrystalline orpolycrystalline silicon. The method also involves forming a conductivepaste layer on the barrier layer. The method also involves forming aconductive layer from the conductive paste layer. The method alsoinvolves forming a contact structure for the semiconductor region of thesolar cell, the contact structure including at least the conductivelayer.

Also disclosed herein are solar cells. In an embodiment, a solar cellincludes a substrate. A polycrystalline silicon layer of an emitterregion is disposed above the substrate. A contact structure is disposedon the polycrystalline silicon layer of the emitter region and includesa conductive layer in contact with a barrier layer disposed on thepolycrystalline silicon layer of the emitter region. The conductivelayer includes a matrix binder having aluminum-containing particlesdispersed therein.

In another embodiment, a solar cell includes a monocrystalline siliconsubstrate. A diffusion region is disposed in the monocrystalline siliconsubstrate. A contact structure is disposed on the diffusion region andincludes a conductive layer in contact with a barrier layer disposed onthe diffusion region. The conductive layer includes a matrix binderhaving aluminum-containing particles dispersed therein.

In a first exemplary cell, a barrier layer is used during thefabrication of contacts, such as back-side contacts, for a solar cellhaving emitter regions formed above a substrate of the solar cell. Forexample, Figure 1A illustrates a cross-sectional view of a portion of asolar cell having contact structures formed on emitter regions formedabove a substrate, in accordance with an embodiment of the presentdisclosure.

Referring to FIG. 1A, a portion of a solar cell 100A includes apatterned dielectric layer 224 disposed above a plurality of n-typedoped polysilicon regions 220, a plurality of p-type doped polysiliconregions 222, and on portions of a substrate 200 exposed by trenches 216.Contact structures 228 are disposed in a plurality of contact openingsdisposed in the dielectric layer 224 and are coupled to the plurality ofn-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222. The materials of, and methods offabricating, the patterned dielectric layer, the plurality of n-typedoped polysilicon regions 220, the plurality of p-type doped polysiliconregions 222, the substrate 200, and the trenches 216 may be as describedbelow in association with FIGS. 2A-2C. Furthermore, the plurality ofn-type doped polysilicon regions 220 and the plurality of p-type dopedpolysilicon regions 222 can, in one embodiment, provide emitter regionsfor the solar cell 100A. Thus, in an embodiment, the contact structures228 are disposed on the emitter regions. In an embodiment, the contactstructures 228 are back contacts for a back-contact solar cell and aresituated on a surface of the solar cell opposing a light receivingsurface (direction provided as 201 in FIG. 1A) of the solar cell 100A.Furthermore, in one embodiment, the emitter regions are formed on a thinor tunnel dielectric layer 202, described in greater detail inassociation with FIG. 2A.

Referring again to FIG. 1A, each of the contact structures 228 includesa conductive layer 130 (also referred to herein as a seed layer) incontact with a barrier layer 250 disposed on the emitter regions of thesolar cell 100A. In an embodiment, the conductive layer 130 includes amatrix binder having aluminum-containing particles dispersed therein.

In accordance with a first aspect of the disclosure, and as described ingreater detail below in association with FIGS. 4A-4D and 5A-5E, in anembodiment, the barrier layer 250 is a metal-containing layer. In onesuch embodiment, the metal-containing layer includes a metal such as,but not limited to, nickel (Ni), titanium (Ti) or tungsten (W).

In accordance with a second aspect of the disclosure, and as describedin greater detail below in association with FIGS. 6A and 6B, in anembodiment, the barrier layer 250 is a tunneling dielectric layer. Inone such embodiment, the tunneling dielectric layer is a thin siliconoxide layer.

In accordance with a second aspect of the disclosure, and as describedin greater detail below in association with FIGS. 7A-7D, in anembodiment, the barrier layer 250 is a metal silicide layer. In one suchembodiment, the metal silicide layer is a nickel (Ni) silicide layer.

With reference to all three of the above described aspects, in anembodiment, the aluminum-containing particles are aluminum/silicon(Al/Si) particles. In another embodiment, however, thealuminum-containing particles are aluminum-only particles. In anembodiment, the contact structure 228 further includes a nickel (Ni) orzinc (Zn) layer 132, or both, disposed on the conductive layer 130. Acopper (Cu) layer 134 is disposed on the Ni or Zn layer 132. However, inanother embodiment, no additional conductive layers are disposed on theconductive layer 130.

In a second exemplary cell, a barrier layer is used during thefabrication of contacts, such as back-side contacts, for a solar cellhaving emitter regions formed in a substrate of the solar cell. Forexample, Figure 1B illustrates a cross-sectional view of a portion of asolar cell having contact structures formed on emitter regions formed ina substrate, in accordance with an embodiment of the present disclosure.

Referring to FIG. 1B, a portion of a solar cell 100B includes apatterned dielectric layer 124 disposed above a plurality of n-typedoped diffusion regions 120, a plurality of p-type doped diffusionregions 122, and on portions of a substrate 100, such as a bulkcrystalline silicon substrate. Contact structures 128 are disposed in aplurality of contact openings disposed in the dielectric layer 124 andare coupled to the plurality of n-type doped diffusion regions 120 andto the plurality of p-type doped diffusion regions 122. In anembodiment, the diffusion regions 120 and 122 are formed by dopingregions of a silicon substrate with n-type dopants and p-type dopants,respectively. Furthermore, the plurality of n-type doped diffusionregions 120 and the plurality of p-type doped diffusion regions 122 can,in one embodiment, provide emitter regions for the solar cell 100B.Thus, in an embodiment, the contact structures 128 are disposed on theemitter regions. In an embodiment, the contact structures 128 are backcontacts for a back-contact solar cell and are situated on a surface ofthe solar cell opposing a light receiving surface, such as opposing atexturized light receiving surface 101, as depicted in FIG. 1B. In anembodiment, referring again to FIG. 1B, each of the contact structures128 includes a conductive layer 130 in contact with a barrier layer 150disposed on the emitter regions (i.e., diffusion regions) of the solarcell 100B. In an embodiment, the conductive layer 130 includes a matrixbinder having aluminum-containing particles dispersed therein.Furthermore, the contact structures 128, the conductive layer 130 andthe barrier layer 150 of Figure 1B may be similar to or the same as thecontact structures 228, the conductive layer 130 and the barrier layer250 described above in association with FIG. 1A.

Although certain materials are described specifically above withreference to FIGS. 1A and 1B, some materials may be readily substitutedwith others with other such embodiments remaining within the spirit andscope of embodiments of the present disclosure. For example, in anembodiment, a different material substrate, such as a group III-Vmaterial substrate, can be used instead of a silicon substrate. Inanother embodiment, silver (Ag) particles or the like can be used in aconductive layer in addition to, or instead of Al or Al/Si particles. Inanother embodiment, plated or like-deposited cobalt (Co) or tungsten (W)can be used instead of or in addition to the Ni layer described above.

Furthermore, the formed contacts need not be formed directly on a bulksubstrate, as was described in FIG. 1B. For example, in one embodiment,contact structures such as those described above are formed onsemiconducting regions formed above (e.g., on a back side of) as bulksubstrate, as was described for FIG. 1A. As a fabrication processexample, FIGS. 2A-2C illustrate cross-sectional views of variousprocessing operations in a method of fabricating solar cells havingcontact structures, in accordance with an embodiment of the presentdisclosure. FIG. 3 is a flowchart 300 illustrating operations in amethod of fabricating a solar cell corresponding to the exemplaryoperations described in association with FIGS. 2B and 2C, in accordancewith an embodiment of the present disclosure.

Referring to FIG. 2A, a method of forming contacts for a back-contactsolar cell includes forming a thin dielectric layer 202 on a substrate200.

In an embodiment, the thin dielectric layer 202 is composed of silicondioxide and has a thickness approximately in the range of 5-50Angstroms. In one embodiment, the thin dielectric layer 202 performs asa tunneling oxide layer. In an embodiment, substrate 200 is a bulkmonocrystalline silicon substrate, such as an n-type dopedmonocrystalline silicon substrate. However, in an alternativeembodiment, substrate 200 includes a polycrystalline silicon layerdisposed on a global solar cell substrate.

Referring again to FIG. 2A, trenches 216 are formed between n-type dopedpolysilicon regions 220 and p-type doped polysilicon regions 222.Portions of the trenches 216 can be texturized to have textured features218, as is also depicted in FIG. 2A. A dielectric layer 224 is formedabove the plurality of n-type doped polysilicon regions 220, theplurality of p-type doped polysilicon regions 222, and the portions ofsubstrate 200 exposed by trenches 216. In one embodiment, a lowersurface of the dielectric layer 224 is formed conformal with theplurality of n-type doped polysilicon regions 220, the plurality ofp-type doped polysilicon regions 222, and the exposed portions ofsubstrate 200, while an upper surface of dielectric layer 224 issubstantially flat, as depicted in FIG. 2A. In a specific embodiment,the dielectric layer 224 is an anti-reflective coating (ARC) layer.

Referring to FIG. 2B, a plurality of contact openings 226 is formed inthe dielectric layer 224. The plurality of contact openings 226 providesexposure to the plurality of n-type doped polysilicon regions 220 and tothe plurality of p-type doped polysilicon regions 222. In oneembodiment, the plurality of contact openings 226 is formed by laserablation. In one embodiment, the contact openings 226 to the n-typedoped polysilicon regions 220 have substantially the same height as thecontact openings to the p-type doped polysilicon regions 222, asdepicted in FIG. 2B.

Referring again to FIG. 2B, and to corresponding operation 302 offlowchart 300, a barrier layer 298 is formed on the exposed portions ofeach of the plurality of n-type doped polysilicon regions 220 and to theplurality of p-type doped polysilicon regions 222.

In accordance with the first aspect of the disclosure, and as describedin greater detail below in association with FIGS. 4A-4D and 5A-5E, in anembodiment, the barrier layer 298 is a metal-containing layer. In onesuch embodiment, the metal-containing layer includes a metal such as,but not limited to, nickel (Ni), titanium (Ti) or tungsten (W). Inanother embodiment, the barrier layer 298 is an insulating layer. In onesuch embodiment, the insulating layer includes a silicon oxide layer. Ina particular such embodiment, the insulating layer further includes apartially recessed silicon nitride layer on the silicon oxide layer.

In accordance with the second aspect of the disclosure, and as describedin greater detail below in association with FIGS. 6A and 6B, in anembodiment, the barrier layer 298 is a tunneling dielectric layer. Inone such embodiment, the term “tunneling dielectric layer” refers to avery thin (e.g., less than approximately 10 nm) dielectric layer,through which electrical conduction can be achieved. The conduction maybe due to quantum tunneling and/or the presence of small regions ofdirect physical connection through thin spots in the dielectric layer.In one embodiment, the tunneling dielectric layer is or includes a thinsilicon oxide layer. In an embodiment, a silicon layer is further formedon the tunneling dielectric layer.

In accordance with the third aspect of the disclosure, and as describedin greater detail below in association with FIGS. 7A-7D, in anembodiment, the barrier layer 298 includes a metal silicide layer. Inone such embodiment, the metal silicide layer is formed by consuming aportion of the silicon from the plurality of n-type doped polysiliconregions 220 and to the plurality of p-type doped polysilicon regions222. In one embodiment, the metal silicide layer is formed by firstforming a nickel silicide layer by plating a nickel (Ni) layer,activating the Ni layer, and annealing the Ni layer to react with theplurality of n-type doped polysilicon regions 220 and to the pluralityof p-type doped polysilicon regions 222. In a particular suchembodiment, subsequent to annealing the Ni layer, any unreacted Ni isremoved.

Referring to FIG. 2C, the method of forming contacts for theback-contact solar cell further includes forming contact structures 228in the plurality of contact openings 226 and coupled to the plurality ofn-type doped polysilicon regions 220 and to the plurality of p-typedoped polysilicon regions 222. Thus, in an embodiment, contactstructures 228 are formed on or above a surface of a bulk N-type siliconsubstrate 200 opposing a light receiving surface 201 of the bulk N-typesilicon substrate 200. In a specific embodiment, the contact structuresare formed on regions (222/220) above the surface of the substrate 200,as depicted in FIG. 2C.

More particularly, referring to operation 304 of flowchart 300, aconductive paste layer (not shown) is formed on the barrier layer 298 ofFIG. 2B. In an embodiment, the conductive paste layer is formed from amixture including aluminum (Al) and/or aluminum/silicon (Al/Si)particles, a liquid binder, and a frit material. In an embodiment, theconductive paste layer is formed by screen printing. Referring again toFIG. 2C and now to corresponding operation 306 of flowchart 300, aconductive layer 299 is formed from the conductive paste layer, e.g., bylaser or thermal anneal. Referring again to FIG. 2C and now tocorresponding operation 308 of flowchart 300, a contact structure 228 isformed to include at least the conductive layer 299. Additionally, in anembodiment, the barrier layer 298 described in association with FIG. 2Bis retained and ultimately disposed between the contact structures 228and the plurality of n-type doped polysilicon regions 220 and theplurality of p-type doped polysilicon regions 222 (not shown). However,in other embodiments, the barrier layer 298 is removed prior to orduring the fabrication of contact structures 228, as is depicted in FIG.2C. Exemplary embodiments of the above operations are provided below.

In accordance with the first aspect of the disclosure, and as describedin greater detail below in association with FIGS. 4A-4D and 5A-5E, in anembodiment, the barrier layer 298 of FIG. 2B is a metal-containinglayer, and the conductive layer 299 is formed in contact with themetal-containing layer. In another embodiment, the barrier layer 298 ofFIG. 2B is an insulating layer, and the conductive layer 299 is formedthrough the insulating layer and in contact with the semiconductorregion (e.g., region 220 or 222) of the solar cell.

In accordance with the second aspect of the disclosure, and as describedin greater detail below in association with FIGS. 6A and 6B, in anembodiment, the barrier layer 298 of FIG. 2B is a tunneling dielectriclayer having a silicon layer formed thereon, and the conductive layer299 is formed by consuming at least a portion of the silicon layer withthe conductive paste layer. In an embodiment, conductive layer 299 isformed in contact with the tunneling dielectric layer.

In accordance with the third aspect of the disclosure, and as describedin greater detail below in association with FIGS. 7A-7D, in anembodiment, the barrier layer 298 of FIG. 2B is a metal silicide layer,and the conductive layer 299 is formed in contact with the metalsilicide layer. In one such embodiment, the conductive paste layer is analuminum (Al) paste layer used to form the conductive layer 299 isformed on a nickel silicide layer.

With reference to all three of the above described aspects, in anembodiment, forming the conductive layer 299 from the conductive pastelayer involves firing the conductive paste layer at a temperature aboveapproximately 500 degrees Celsius for a duration of at leastapproximately 10 minutes. Furthermore, it is to be appreciated that theconductive layer 299 may be used on its own to form contact structures);in such cases, the conductive layer 299 may still be referred to hereinas a seed layer. Alternatively, completion of the contact structures 228further involves plating a nickel (Ni) layer on the conductive layer299, and electroplating a copper (Cu) layer on the Ni layer, e.g., toform structures such as those described in association with FIGS. 1A and1B. In yet another alternative embodiment, forming the contactstructures 228 further involves electroplating a copper (Cu) layerdirectly on the conductive layer 299. Generally, as used in embodimentsthroughout, a formed paste layer (e.g., a deposited paste formed byprinting) can further include a solvent for ease of delivery.

As described briefly above, and in greater detail below in associationwith FIGS. 4A-4D and 5A-5E, in a first aspect of the present disclosure,barrier layers are used to enable high temperature firing of printedmetal for solar cell contact formation. One or more embodimentsaddresses both adhesion and contact resistance issues for printed metalsby placing a continuous barrier layer between a paste layer and amonocrystalline or polysilicon region to prevent or retard diffusion ofthe polysilicon into the paste (e.g., to prevent silicon consumption).Furthermore, in some cases, a continuous metal semiconductor orsilicide-semiconductor interface is provided to improve contactresistance.

To provide further context, printed seed processing typically involvesuse of an aluminum-silicon paste which is fired below approximately 560degrees Celsius in order to prevent diffusion of polysilicon (e.g., froman emitter region) into the paste. Firing time may be limited toapproximately 30 minutes in order to prevent degradation of lifetime.Such an upper limit on the firing temperature and time can limit thedegree of particle sintering which is possible, which in turn limits thedegree of adhesion. Paste to silicon contact resistance may also limitedby relatively small area of point contacts that can be made betweenmostly spherical particles in the paste and the planar exposedpolysilicon in the contact openings.

In a first example, FIGS. 4A-4D illustrate cross-sectional views ofvarious processing operations in another method of fabricating solarcells having contact structures, in accordance with an embodiment of thepresent disclosure.

Referring to FIG. 4A, a silicon substrate or region 402 has aninsulating layer 404 formed thereon. In an embodiment, the insulatinglayer is a silicon oxide layer having a thickness of approximately 100Angstroms. In one such embodiment, an additional insulating layer 406 isdisposed thereon, such as a silicon nitride layer.

Referring to FIG. 4B, the additional insulating layer 406 is recessed toform a trench 408 above the silicon substrate or region 402. In one suchembodiment, the additional insulating layer 406 is only partiallyrecessed, as shown. However, in another embodiment, the trench 408 isformed to expose the insulating layer 404.

Referring to FIG. 4C, a metal paste 410 is formed in the trench 408 andis separated from the silicon substrate or region 402 by at least theinsulating layer 404. In an embodiment, the metal paste 410 is printedand includes Al or Al/Si particles, as described above.

Referring to FIG. 4D, the metal paste 410 is fired (e.g., thermallyannealed or laser annealed) to form a conductive layer 412. Theconductive layer 412 breaks through the insulating layer 404 to directlycontact the silicon substrate or region 402. In accordance with anembodiment of the present disclosure, the conductive layer 412 is usedto fabricate a conductive contact structure for a solar cell.

In a second example, FIGS. 5A-5E illustrate cross-sectional views ofvarious processing operations in another method of fabricating solarcells having contact structures, in accordance with an embodiment of thepresent disclosure.

Referring to FIG. 5A, a silicon substrate or region 502 has aninsulating layer 504 formed thereon. In an embodiment, the insulatinglayer is a silicon oxide layer having a thickness of approximately 100Angstroms. In one such embodiment, an additional insulating layer 506 isdisposed thereon, such as a silicon nitride layer.

Referring to FIG. 5B, both the additional insulating layer 506 and theinsulating layer 504 are recessed to provide have a trench 508 formedtherein, exposing the top surface of the silicon substrate or region502.

Referring to FIG. 5C, a metal layer 509 is formed in the trench 508. Inan embodiment, the metal layer 509 is formed selectively on the topsurface of the silicon substrate or region 502. In one embodiment, themetal layer 509 includes tungsten metal deposited via a selectivechemical vapor deposition (CVD) process. Tungsten can be selectivelydeposited on silicon according to the reaction 2WF₆+3Si->2W+3SiF₄ atpressures of a few Torr. The reaction is favored over deposition of Wonto SiO₂ and/or silicon nitride (SiNx). The reaction is self limitingat approximately 250 Angstroms of tungsten deposition on the exposedsilicon. In one embodiment, SiH₄ is mixed into the process gas andthicker tungsten layers are grown while maintaining good selectivity. Ina particular such embodiment, a process window for SiH₄/WF₆ processchemistry involves a substrate temperature approximately in the range of280-350 degrees Celsius, a total pressure of approximately 100 mTorr, aSiH₄/WF₆ flow ratio less than approximately 0.6, and H₂ carrier gas flowrate of approximately 1000 sccm. In another embodiment, the metal layer509 includes titanium silicide deposited via a selective CVD process.Selective deposition of titanium disilicide on silicon can be performedusing TiCl₄ and SiH₄ as precursor gasses to achieve low resistancecontacts to silicon.

Referring to FIG. 5D, a metal paste 510 is formed in the trench 408 andon the metal layer 509. In an embodiment, the metal paste 510 is printedand includes Al or Al/Si particles, as described above.

Referring to FIG. 5E, the metal paste 510 is fired (e.g., thermallyannealed or laser annealed) to form a conductive layer 512. In theillustrated embodiment, the conductive layer 512 does not directlycontact the silicon substrate or region 502 because the interveningmetal layer 509 remains. In an embodiment, the paste is fired above themelting point of the particles to enable good particle to particle andparticle to SiNx adhesion. Where a tungsten or titanium silicide barrierlayer is used, a physical barrier is created to prevent polysilicon fromdiffusing into the aluminum paste matrix. In accordance with anembodiment of the present disclosure, the conductive layer 512 and themetal layer 509 together are used to fabricate a conductive contactstructure for a solar cell.

As also described briefly above, and in greater detail below inassociation with FIGS. 6A and 6B, in a second aspect of the presentdisclosure, a tunnel dielectric barrier layer is implemented for contactformation based on printable metal paste. In an exemplary embodiment, atunneling dielectric layer is used as a barrier layer to ultimatelyprevent or reduce silicon pitting when firing an aluminum-based metalpaste. In general, dielectric layers, such as oxides, prevent pittingbelow the dielectric, but by their very nature are not employed as ametallization barrier due to their electrically insulating properties.In an embodiment, by using a thin dielectric layer, charge carriers cantunnel through the dielectric such that current can pass and anelectrical circuit can be closed. Even at such small thicknesses,however, the dielectric layer can maintain the diffusion barrierproperties which prevent/reduce silicon pitting.

To provide further context, use of aluminum particles within a printablepaste is a common approach for depositing a metal solar cell contact instandard front-contact silicon solar cells. When fired (i.e., annealed)to temperatures as high as approximately 550 degrees Celsius in order tosinter the aluminum particles and form contact between the aluminumpaste and the silicon substrate, the aluminum can react with the siliconto form a eutectic alloy of aluminum and silicon, with the silicon asthe diffusing species. Such consumption of silicon is commonly referredto as pitting or spiking. In standard cells, this effect is notdetrimental enough to be significant, but in high efficiency cells thepitting can lead to a significant reduction in cell performance. Oneapproach for addressing the above issue is to alloy the aluminumparticles with silicon in order to decrease the pitting effect. However,even at silicon concentrations greater than the eutectic, pitting canstill observed due to the formation of precipitates and transientbehavior of the silicon concentration within aluminum while heating thesystem. Another solution to avoid pitting includes inhibiting a firingor annealing temperature from activating a significant amount of thepitting reaction. However, even at temperatures below the Al/Si eutecticformation temperature of 577 degrees Celsius, spiking can still occur.Additionally, at such lower temperatures, it can be difficult to makeelectrical contact between the aluminum particles and the siliconsubstrate or regions, and still achieve the necessary adhesion of thepaste to the substrate or region. Another influence on the pittingreaction can include particle size, which has an effect both on thesilicon concentration gradients within particles and on the reaction ofthe particles with the substrate to form contacts. By combining theseapproaches, the pitting reaction can be managed, but the combination canstill have shortcomings which limit the processing window and/or cellperformance. While these approaches can reduce the effect of pitting,none of these options prevents pitting completely. The disclosedstructures and techniques can improve on those shortcomings.

In a first example, FIG. 6A illustrates cross-sectional views of variousprocessing operations in another method of fabricating solar cellshaving contact structures, in accordance with an embodiment of thepresent disclosure.

Referring to part (Ia) of FIG. 6A, in the fabrication of a solar cellhaving emitter regions formed in a monocrystalline silicon substrate602, a tunnel dielectric layer 604 is formed on the monocrystallinesilicon substrate 602. A metal paste layer 608 is then formed on thetunnel dielectric layer 604. Referring to part (Ib) of FIG. 6A, inanother embodiment, a polycrystalline silicon layer 606 is formed on thetunnel dielectric layer 604, and the metal paste layer 608 is formed onthe polycrystalline silicon layer 606.

Referring to part (II) of FIG. 6A, for either case (Ia) or (Ib), themetal paste layer 608 is fired (e.g., by thermal or laser anneal) toform a conductive layer 612 on the tunnel dielectric layer 604. It is tobe appreciated that, for the case of part (Ib), the conductive layer 612of part (II) will include therein silicon from the polycrystallinesilicon layer 606. In accordance with an embodiment of the presentdisclosure, the conductive layer 612 is used to form a conductivecontact structure for a back contact solar cell.

In a second example, FIG. 6B illustrates cross-sectional views ofvarious processing operations in another method of fabricating solarcells having contact structures, in accordance with an embodiment of thepresent disclosure.

Referring to part (Ia) of FIG. 6B, emitter regions are/will be formedusing a first polycrystalline silicon layer 655 disposed on a firsttunnel dielectric layer 653, the first tunnel dielectric layer 653disposed on a silicon substrate 652. A second tunnel dielectric layer654 is then formed on the first polycrystalline silicon layer 655. Ametal paste layer 658 is then formed on the second tunnel dielectriclayer 654. Referring to part (Ib) of FIG. 6B, in another embodiment, asecond polycrystalline silicon layer 656 is formed on the second tunneldielectric layer 654, and the metal paste layer 658 is formed on thesecond polycrystalline silicon layer 656.

Referring to part (II) of FIG. 6B, for either case (Ia) or (Ib), themetal paste layer 658 is fired (e.g., by thermal or laser anneal) toform a conductive layer 662 on the second tunnel dielectric layer 654.It is to be appreciated that, for the case of part (Ib), the conducivelayer 662 of part (II) will include therein silicon from the secondpolycrystalline silicon layer 656. In accordance with an embodiment ofthe present disclosure, the conductive layer 662 is used to form aconductive contact structure for a back contact solar cell.

Accordingly, referring again to FIGS. 6A and 6B, in an embodiment a thindielectric layer is used to prevent pitting of an underlying siliconregion or layer and yet allow current to pass through the thindielectric layer. The ability for current to pass through the dielectriclayer allows for current transport out of the cell. By employing such apitting barrier, the temperature for firing can be increased to optimizepaste sintering for electrical performance and adhesion, without aconcern for pitting. Since the pitting mechanism is blocked, thealuminum does not need to be alloyed, which reduces the complexity andcost of the paste. Furthermore, since the particles can be melted, thereis little to no concern over particle size and number of point contactsto the substrate surface. The melting effect also increases adhesionwhich can negate the need for specialized binders or frits within thepaste to promote adhesion in the lower temperature systems.

In an embodiment, referring again to part (Ib) of FIGS. 6A and 6B, inthe case that a sacrificial polycrystalline silicon is used in thefabrication of a contact structure including a tunnel dielectric layer,the sacrificial polycrystalline silicon is used to protect the tunneldielectric layer from damage during processing and/or to improve thewettability of the metal paste on the tunnel dielectric layer. Forexample, if the tunnel dielectric layer is a thin silicon oxide layer,an overlying polycrystalline of amorphous silicon layer can preventfurther uncontrolled oxidation of the thin oxide layer, which mightotherwise compromise the thin oxide interfaces or uncontrollablyincrease the thin oxide thickness. In an embodiment, the tunneldielectric layer 604 or 654 is deposited by a chemical vapor deposition(CVD) method, such as but not limited to plasma enhanced CVD (PECVD),low pressure CVD (LPCVD) or atmospheric pressure CVD (APCVD). Atomiclayer deposition (ALD) is another option for tight control in thedeposition of thin dielectric films. The tunnel dielectric layer mayalso be grown as a thin oxide film, using chemical oxidation or thermaloxidation. In further embodiments, the thin silicon oxide layer issubjected to nitridation. In addition to the use of a silicon oxide as atunnel dielectric layer, silicon oxynitrides or aluminum oxides may alsobe used.

As also described briefly above, and in greater detail below inassociation with FIGS. 7A-7D, in a third aspect of the presentdisclosure, metallization of back contact solar cells is performed byreorganizing/changing a fabrication process flow of the metallizationsequence in order to take advantages of one or more technologicalphenomenon. For example, in an embodiment, metallization is performed byforming a nickel (Ni) silicide layer as an interface between a metalstack of a conductive contact structure and the corresponding siliconsubstrate or region to reduce the contact and device series resistances.In another embodiment, associated critical risks of cell fabrication areswitched from the paste development/formulation to the platingdevelopment and silicide-based contact formation. In another embodiment,improved efficiency is achieved by the formation of, and ultimateretentions of, Ni silicide as compared to an Al—Si contact formed at atemperature below the eutectic point. In another embodiment, a standardAl-only paste can be used at low cost, instead of using a higher costAl—Si particle based paste.

In an example, FIGS. 7A-7D illustrate cross-sectional views of variousprocessing operations in another method of fabricating solar cellshaving contact structures, in accordance with an embodiment of thepresent disclosure.

Referring to FIG. 7A, an insulating layer 704 is formed above a siliconsubstrate or region 702. Contact openings 706 are formed in theinsulating layer 704, exposing the top surface of the silicon substrateor region 702. In an embodiment, the insulating layer 704 includes asilicon nitride layer, a silicon oxide layer, or both. In an embodiment,the contact openings 706 are formed by laser ablation or by alithography and etch process.

Referring to FIG. 7B, a metal layer 708 is formed in the contactopenings 706, on the exposed top surface of the silicon substrate orregion 702. In an embodiment, the metal layer 708 is a thin nickel layer(e.g., between approximately 0.1 to 2 microns) formed by electrolessplating. In one such embodiment, an activation operation is usedinvolving an approximately 4% hydrofluoric acid (HF) exposure forseveral seconds.

Referring again to FIG. 7B, an annealing operation is performed to forma metal silicide material 710 upon reaction of the metal layer 708 andsilicon from the silicon substrate or region 702. In an embodiment, theanneal is performed in an atmosphere suitable for forming silicides,such as a forming gas anneal atmosphere, nitrogen or air. At this stage,any nickel that is not reacted (i.e., nickel not used in the formationof a silicide) can then be removed, e.g., by wet etching. It is to beappreciated that more than one metal deposition process may be performedto ultimately provide a suitable metal silicide layer. In anotherembodiment, a nickel layer is formed on, or is retained on, the metalsilicide layer.

Referring to FIG. 7C, a metal paste 712 is formed on the insulatinglayer 704 and in contact with the metal silicide material 710 formed inthe contact openings 706. In an embodiment, the metal paste 410 isprinted and includes Al or Al/Si particles, as described above. In anembodiment, a single line of metal paste contacts the metal silicidematerial 710 of a plurality of the contact openings 706. In anembodiment, the metal of the metal paste 712 is pure aluminum and thepaste contains specific binders suitable for bonding the aluminumparticles to the insulating layer 704 and to the metal silicide 710. Inone such embodiment, a binder such as, but not limited to, ZnOx, BiOx,SnOx, or an inorganic polymer such as phenyl(methyl) silsesquioxane isused. In an embodiment, the paste is then dried and fired (e.g.,thermally annealed or laser annealed) with a specific temperatureprofile to form a conductive layer 713. The drying and firing may alsoburn organics, solvents and reach an acceptable sheet resistance forfurther plating processes. The temperature may be determined by thecomposition of the paste and the effect on the nickel and/or nickelsilicide layers. It is to be appreciated that, in another embodiment,the annealing to form a metal silicide and the paste firing may beperformed in a single operation upon deposition of the metal paste.

Referring to FIG. 7D, conductive contact structures 714 are fabricatedfor a solar cell from the conductive layer 713, and includes the metalsilicide material 710 (as retained in the openings 706. In anembodiment, the conductive contact structures 714 are formed byperforming an electroless zinc deposition (i.e., to form zincate) toactivate the aluminum paste prior to a copper plating operation. Copperplating is then performed followed by a tin plating operation and/or anorganic solder preservative (OSP) deposition process for furthersoldering.

Thus, approaches for the metallization of solar cells and the resultingsolar cells have been disclosed.

Although specific embodiments have been described above, theseembodiments are not intended to limit the scope of the presentdisclosure, even where only a single embodiment is described withrespect to a particular feature. Examples of features provided in thedisclosure are intended to be illustrative rather than restrictiveunless stated otherwise. The above description is intended to cover suchalternatives, modifications, and equivalents as would be apparent to aperson skilled in the art having the benefit of this disclosure.

The scope of the present disclosure includes any feature or combinationof features disclosed herein (either explicitly or implicitly), or anygeneralization thereof, whether or not it mitigates any or all of theproblems addressed herein. Accordingly, new claims may be formulatedduring prosecution of this application (or an application claimingpriority thereto) to any such combination of features. In particular,with reference to the appended claims, features from dependent claimsmay be combined with those of the independent claims and features fromrespective independent claims may be combined in any appropriate mannerand not merely in the specific combinations enumerated in the appendedclaims.

In an embodiment, a method of fabricating a solar cell involves forminga barrier layer on a semiconductor region disposed in or above asubstrate, the semiconductor region including monocrystalline orpolycrystalline silicon. The method also involves forming a conductivepaste layer on the barrier layer. The method also involves forming aconductive layer from the conductive paste layer. The method alsoinvolves forming a contact structure for the semiconductor region of thesolar cell, the contact structure including at least the conductivelayer.

In one embodiment, forming the barrier layer involves forming aninsulating layer.

In one embodiment, forming the insulating layer involves forming asilicon oxide layer.

In one embodiment, forming the insulating layer further involves formingand partially recessing a silicon nitride layer on the silicon oxidelayer.

In one embodiment, forming the conductive layer from the conductivepaste layer involves forming the conductive layer through the insulatinglayer and in contact with the semiconductor region of the solar cell.

In one embodiment, forming the barrier layer involves forming ametal-containing layer.

In one embodiment, forming the metal-containing layer involves forming alayer including a metal such as, but not limited to, nickel (Ni),titanium (Ti) or tungsten (W).

In one embodiment, forming the conductive layer from the conductivepaste layer involves forming the conductive layer in contact with themetal-containing layer.

In one embodiment, forming the barrier layer involves forming atunneling dielectric layer.

In one embodiment, forming the tunneling dielectric layer involvesforming a thin silicon oxide layer.

In one embodiment, forming the barrier layer further involves forming asilicon layer on the tunneling dielectric layer.

In one embodiment, forming the conductive layer involves consuming atleast a portion of the silicon layer with the conductive paste layer.

In one embodiment, forming the conductive layer from the conductivepaste layer involves forming the conductive layer in contact with thetunneling dielectric layer.

In one embodiment, forming the barrier layer involves forming a metalsilicide layer including silicon from the semiconductor region.

In one embodiment, forming the metal silicide layer involves forming anickel silicide layer by plating a nickel (Ni) layer, activating the Nilayer, and annealing the Ni layer to react with the semiconductorregion.

In one embodiment, the method further involves, subsequent to annealingthe Ni layer to react with the semiconductor region, removing anyunreacted Ni prior to forming the conductive paste layer.

In one embodiment, forming the conductive paste layer involves formingan aluminum (Al) paste layer on the nickel silicide layer.

In one embodiment, forming the conductive layer from the conductivepaste layer involves firing the conductive paste layer at a temperatureabove approximately 500 degrees Celsius for a duration of at leastapproximately 10 minutes.

In one embodiment, forming the conductive paste layer involves forming amixture including aluminum/silicon (Al/Si) particles, a liquid binder,and a frit material.

In one embodiment, forming the conductive paste layer involves screenprinting the conductive paste layer.

In one embodiment, forming the contact structure further involvesplating a first metal layer on the conductive layer, and plating asecond metal layer on the first metal layer.

In an embodiment, a solar cell includes a substrate. A polycrystallinesilicon layer of an emitter region is disposed above the substrate. Acontact structure is disposed on the polycrystalline silicon layer ofthe emitter region and includes a conductive layer in contact with abarrier layer disposed on the polycrystalline silicon layer of theemitter region. The conductive layer includes a matrix binder havingaluminum-containing particles dispersed therein.

In one embodiment, the barrier layer is a metal-containing layer.

In one embodiment, the metal-containing layer comprises a metal such as,but not limited to, nickel (Ni), titanium (Ti) and tungsten (W).

In one embodiment, the barrier layer is a tunneling dielectric layer.

In one embodiment, the tunneling dielectric layer includes a thinsilicon oxide layer.

In one embodiment, the barrier layer is a metal silicide layer.

In one embodiment, the metal silicide layer is a nickel (Ni) silicidelayer.

In one embodiment, the aluminum-containing particles arealuminum/silicon (Al/Si) particles.

In one embodiment, the aluminum-containing particles are aluminum-onlyparticles.

In one embodiment, the contact structure further includes a nickel (Ni)or zinc (Zn) layer, or both, disposed on the conductive layer, and acopper (Cu) layer disposed on the Ni or Zn layer.

In one embodiment, the solar cell is a back-contact solar cell.

In an embodiment, a solar cell includes a monocrystalline siliconsubstrate. A diffusion region is disposed in the monocrystalline siliconsubstrate. A contact structure is disposed on the diffusion region andincludes a conductive layer in contact with a barrier layer disposed onthe diffusion region. The conductive layer includes a matrix binderhaving aluminum-containing particles dispersed therein.

In one embodiment, the barrier layer is a metal-containing layer.

In one embodiment, the metal-containing layer includes a metal such as,but not limited to, nickel (Ni), titanium (Ti) or tungsten (W).

In one embodiment, the barrier layer is a tunneling dielectric layer.

In one embodiment, the tunneling dielectric layer includes a thinsilicon oxide layer.

In one embodiment, the barrier layer is a metal silicide layer.

In one embodiment, the metal silicide layer is a nickel (Ni) silicidelayer.

In one embodiment, the aluminum-containing particles arealuminum/silicon (Al/Si) particles.

In one embodiment, the aluminum-containing particles are aluminum-onlyparticles.

In one embodiment, the contact structure further includes a nickel (Ni)or zinc (Zn) layer, or both, disposed on the conductive layer, and acopper (Cu) layer disposed on the Ni or Zn layer.

In one embodiment, the solar cell is a back-contact solar cell.

What is claimed is:
 1. A method of fabricating a solar cell, the methodcomprising: forming a silicon oxide layer on a semiconductor regiondisposed in or above a substrate, the semiconductor region comprisingmonocrystalline or polycrystalline silicon; forming a silicon nitridelayer on the silicon oxide layer; forming a trench into but not throughthe silicon nitride layer by only partially recessing a portion of thesilicon nitride layer; forming a conductive paste layer in the trench;forming a conductive layer from the conductive paste layer, whereinforming the conductive layer comprises extending the trench through thesilicon nitride layer and through the silicon oxide layer to expose thesemiconductor region; and forming a contact structure in directelectrical contact with the semiconductor region of the solar cell, thecontact structure comprising at least the conductive layer.
 2. Themethod of claim 1, wherein forming the silicon oxide layer comprisesforming the silicon oxide layer having a thickness of approximately 100Angstroms.
 3. The method of claim 1, wherein forming the conductivepaste layer comprises forming a metal paste comprising Al or Al/Siparticles.
 4. The method of claim 1, wherein forming the conductivelayer from the conductive paste layer comprises laser annealing theconductive paste layer.
 5. The method of claim 1, wherein forming theconductive layer from the conductive paste layer comprises thermallyannealing the conductive paste layer.
 6. The method of claim 1, whereinforming the silicon oxide layer on the semiconductor region comprisesforming the silicon oxide layer on a semiconductor region disposed inthe substrate.
 7. The method of claim 1, wherein forming the siliconoxide layer on the semiconductor region comprises forming the siliconoxide layer on a semiconductor region disposed above the substrate.
 8. Amethod of fabricating a solar cell, the method comprising: forming asilicon nitride layer above a semiconductor region disposed in or abovea substrate, the semiconductor region comprising monocrystalline orpolycrystalline silicon; forming a trench into but not through thesilicon nitride layer by only partially recessing a portion of thesilicon nitride layer; forming a conductive paste layer in the trench;forming a conductive layer from the conductive paste layer, whereinforming the conductive layer comprises extending the trench through thesilicon nitride layer to expose the semiconductor region; and forming acontact structure in direct electrical contact with the semiconductorregion of the solar cell, the contact structure comprising at least theconductive layer.
 9. The method of claim 8, wherein forming theconductive paste layer comprises forming a metal paste comprising Al orAl/Si particles.
 10. The method of claim 8, wherein forming theconductive layer from the conductive paste layer comprises laserannealing the conductive paste layer.
 11. The method of claim 8, whereinforming the conductive layer from the conductive paste layer comprisesthermally annealing the conductive paste layer.
 12. The method of claim8, wherein forming the silicon nitride layer above the semiconductorregion comprises forming the silicon nitride layer above a semiconductorregion disposed in the substrate.
 13. The method of claim 8, whereinforming the silicon nitride layer above the semiconductor regioncomprises forming the silicon nitride layer above a semiconductor regiondisposed above the substrate.